The present invention relates to an arrangement and a method for generating hydrogen from hydrocarbon fuel. The arrangement comprises a fuel reformer for reforming hydrocarbon fuel to a hydrogen rich gas comprising at least hydrogen, carbon monoxide and possibly unconverted fuel compounds. The hydrogen gas produced according to the present invention can advantageously be used to operate fuel cells. The present invention also relates to an auxiliary power unit (APU) comprising such an arrangement.
The conversion of hydrocarbon fuels to a hydrogen rich product gas is a central process step in fuel processors as for instance those used in APU systems. APU systems can advantageously be used in vehicles of all kinds as well as in other applications which have a need for off-grid generation of electricity from a main fuel source. For vehicle power trains, liquid or gaseous hydrocarbon fuels like gasoline, kerosene and diesel, but also biogas or other biofuel, can be utilized. It is therefore straightforward, also to use these fuels for the respective APU systems.
For use in APU systems, several fuel cell technologies are appropriate. These are for instance low temperature proton exchange membrane fuel cells (PEMFC), high temperature fuel cells, e.g. molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC). Since high temperature fuel cells can tolerate carbon monoxide (CO) in the feed gas, the hydrogen rich product gas of the fuel reformer reactor can be supplied directly to the fuel cells. In low temperature fuel cells like PEMFC on the other hand, carbon monoxide poisons the catalysts in the electrode of the anode, and, therefore, the CO content in the hydrogen rich product gas of the reformer needs to be reduced prior to entering the fuel cell. This gas treatment ordinarily comprises at least a water-gas shift reactor and/or a final CO remover step, preferably by oxidation, methanation or metal membranes.
The water-gas shift reaction (WGS) is a chemical reaction in which carbon monoxide reacts with water to form carbon dioxide and hydrogen:CO+H2O→CO2+H2 Thereby, the carbon monoxide amount in the hydrogen rich gas produced by the fuel reformer can be reduced.
For example, US 2007/0287038 describes a method for reforming hydrocarbon fuel for generating hydrogen, wherein after a fuel reformer, which produces the hydrogen rich gas, a gas clean-up reactor is used for removing unwanted by-products such as carbon monoxide. The gas clean-up reactor has a carrier material which is coated with noble metals such as Pt, Ru, Rh and Pd, which react with carbon monoxide, so that the gas stream is purified. Other types of CO oxidizers are described in US 2004/0043343 and WO 2005/107001.
Since most vehicles use liquid hydrocarbons like gasoline, kerosene, or diesel to run their engines, also sulphur compounds, like thiols, thiophenes, organic sulphur compounds or disulphides need to be taken into account. They pose major problems for all catalysts in the system, i.e. the catalysts used for reforming as well as the catalysts in the fuel cell. Therefore, most suppliers for the fuel cells recommend that the total sulphur amount stays well below 1 ppmw (parts per million by weight) in the feed gas. This in turn requires in practice sulphur concentrations in the hydrocarbon fuel fed to the reformer to be in the range of 1 to 10 ppmw, or less.
The removal of sulphur from liquid hydrocarbons down to the aforementioned levels is not trivial and requires severe process conditions, especially for the desulphurization of diesel, which increases the costs for the fuel.
A further problem in the current reformer technology is the conversion of aromatic compounds and long chained carbon compounds comprised in the liquid hydrocarbon fuel. Aromatic compounds can also be formed during the reforming process if the required operating conditions of the reforming process are not or not perfectly met. Mostly, the reforming is performed according to the chemical reactions:CnHm+n-H2O→n-CO+(m/2+n)η2 CO+H2O→CO2+H2 with m, n integer numbers and m>2 and n>1.
Usually, it is not possible to convert the aromatic compounds and long chained carbon compounds in the reformer completely to the hydrogen rich gas, namely hydrogen, carbon dioxide, carbon monoxide, and water, so that the hydrogen rich gas leaving the reformer also comprises unconverted fuel compounds, the so-called fuel slip. This fuel slip is a problem for subsequent clean-up steps of the gas from sulphur compounds and other inorganic compounds since this fuel slip will act as a poison for the fuel cell downstream of the clean-up process.
To remove the fuel slip, it is known to cool the fuel stream so that the unwanted compounds of the fuel slip condense out and can be removed from the hydrogen rich gas. Disadvantageously, this method does not provide a total removal of the fuel slip and lowers the total efficiency of the system. Further, the fuel slip due to its poisoning character contributes to an unwanted premature aging of the fuel cell exposed to it.
It is desirable to provide an arrangement and a method for generating hydrogen from hydrocarbon fuels, which also in parallel provides the possibility to considerably reduce the amount of fuel slip in the hydrogen rich gas. It is also desirable to provide an arrangement and a method for generating hydrogen from hydrocarbon fuels, which can be used in an APU system.
Aspects of the present invention include an arrangement, an auxiliary power unit, as well as a method.
The present invention is based, according to an aspect thereof, on the idea to arrange a molten salt reactor, particularly a molten carbonate salt reactor, in connection with the fuel reformer, so that the hydrogen rich gas produced by the fuel reformer can be led into the molten salt reactor. The molten salt reactor can be arranged downstream of the fuel reformer, but it is also possible to integrate the molten salt reactor into the fuel reformer, or even arrange the molten salt reactor upstream of the fuel reactor. An upstream arrangement is in particular advantageous, if a fuel reformer with a recirculation possibility of the hydrogen gas into the fuel reformer for further catalytic conversion is used.
It is preferred to use molten carbonate salt, but also other suitable molten salts and/or molten salt mixtures can be used. A salt or a salt mixture is suitable in case it shows catalytic properties and is not harmful to the other components in the system. Other examples can be metal-nitride salts like MeNO3, MeSO4 etc. Chloride salts on the other hand should preferably not be used due to their corrosive and poisonous properties.
It is particularly advantageous to use a so called tank type molten salt reactor, where the molten salt is contained in a tank or container as a liquid which the hydrogen rich gas is brought into thereby enabling in a very efficient way the hydrogen rich gas molecules to come into contact with the molten salt molecules of the liquid.
In a further preferred embodiment, the molten salt reactor is designed as a so called plug flow reactor comprising a porous structure e.g. in form of a membrane, which is impregnated or penetrated with molten salt. Generally, in a plug flow reactor, one or more fluid reagents are pumped through a pipe or tube. The chemical reaction proceeds as the reagents travel through the tube. In this type of reactor, the changing reaction rate creates a gradient with respect to distance traversed i.e. at the inlet to the reactor the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate correspondingly slows down.
Molten salt reactors and especially molten carbonate salt reactors have proven useful for the destruction of organic compounds, since the molten salt behaves like a liquid and has the properties to split organic compounds into carbon monoxide, carbon dioxide, hydrogen and water. Additionally, molten carbonate salt is also able to react with sulphur and produces non-soluble and non-gaseous sulphur compounds that—depending on the mechanical design of the container—can easily be kept inside the molten salt container.
It is further advantageous that molten salt reactors are also known to water-gas shift the hydrogen rich gas, i.e. to further react carbon monoxide contained in the hydrogen rich gas with water to form carbon dioxide and hydrogen according to the water-gas shift reaction CO+H2O→CO2+H2. Thereby, also a first high temperature water-gas shift reactor for the removal of carbon monoxide can be replaced by the molten salt reactor.
These properties simplify the clean-up process by reducing the numbers of reactors necessary for cleaning the hydrogen rich gas. Additionally, carbonate salt such as the sodium carbonate is a very cheap substance, which can be easily replaced in case the salt gets too contaminated during operation of the fuel reformer by the sulphur compounds.
Whether the salt or the salt mixture is contaminated can be easily detected by sensing the conductivity and/or the potential of the molten salt or the molten salt mixture. The conductivity and/or the potential of the molten salt or the molten salt mixture depend on the amount of molten salt not contaminated with sulphur. Therefore, a contamination of the molten salt can be detected, in case the conductivity or the potential drops, significantly. In a further embodiment, the molten salt reactor further comprises a sensor, e.g. an ampere-meter, a voltmeter and/or resistance measuring device, sensing the contamination of the molten salt.
It is further necessary to keep the molten salt reactor in the range of the melting point temperature of the used salt or the used salt mixture. Preferably, the temperature of the molten salt reactor is above the melting point of the salt or the salt mixture, but a catalytic reaction of the salt with the sulphur compounds already takes place before the molten salt reactor reaches a temperature above the melting point. This is due to the fact that even before the temperature of the melting point has been reached some of the salt crystals are already molten and can react with the sulphur compounds. Therefore, the catalytic function of the salt or the salt mixture is only almost zero, if the salt is not melted completely.
In general, the melting point of the salts or the salt mixtures used in the reactor depends on the choice of the specific salt or on the choice of specific salts for the salt mixture used and their respective percentage (%) in said salt mixture. A salt mixture can have a melting point lower than the melting points of the individual salts which the salt mixture is composed of. For example sodium carbonate salt (Na2CO3) has a melting point of 851° C. and Lithium carbonate salt (Li2CO3) has a Melting point of 723° C., wherein an eutectic mixture of sodium carbonate salt and lithium carbonate salt ((LiZNa)2CO3) has a melting point of 480° C. An eutectic mixture is a mixture at such proportions that the melting point of the mixture is as low as possible, and that furthermore all the constituents crystallize simultaneously at this temperature from molten liquid solution.
In a preferred embodiment of the invention, the molten salt reactor is placed after the fuel reformer and before the other clean-up steps of the system. Since the reactor has to be at a temperature that is above the melting point of the salt or salt mixture used therein, it is further advantageous to arrange it in close proximity to the fuel reformer and to use the waste heat of the fuel reformer generated during the fuel reforming process for heating of the reactor.
As mentioned above, the molten salt reactor can be arranged in many ways like a tank reactor in form of a container containing the salt and with an inlet and an outlet where the gas is pushed through the molten salt, or like a plug flow type reactor where the molten salt is trapped in a porous matrix structure by capillary forces and the gas is pushed through that porous structure. Such a porous structure can be made from a porous metal, such as porous Nickel, or any suitable porous ceramic, such as porous Lithium Aluminium Oxide.
If a molten salt reactor with a container is used, the container preferably is made from a material, which cannot be corroded by the molten salt. Such material can be one of stainless steel, nickel or ceramic compounds as for instance aluminium oxide. Further, both the inlet and the outlet of such a container can be made from the same or a different non-corrosive material as the container itself. If, alternatively, the salt is encapsulated within a suitable porous structure inside the reactor that traps the salt preferable by capillary forces, the salt is prevented from contacting any other parts of the reactor and/or reformer.
In a further preferred embodiment of the tank type reactor the molten salt reactor inlet is arranged below the molten salt level, wherein the molten salt reactor outlet is arranged above the molten salt level.
By using the inventive arrangement, the sulphur trap and a high temperature water-gas shift reactor can be replaced by the molten salt reactor, whereby the cleanup process of the hydrogen rich gas can be simplified.
In a further preferred embodiment, the inventive arrangement is used in an auxiliary power unit (APU) that can be used for instance for all kinds of vehicles. Vehicles can be land-driven vehicles such as cars, trucks or construction equipment as well as airplanes and all kinds of ships or boats.
Further advantages and preferred embodiments are described in the claims, the description and the FIGURE. Further, it should be noted that the scope of the claimed invention is defined by the appending claims only, and the preferred embodiment shown in the FIGURE is only exemplarily.